Tonometry is the noninvasive measurement of the intraocular pressure of the eye and is commonly employed for the detection of glaucoma. The pressure within the healthy eye of a normal adult will typically be about 17 mm Hg (mercury). Glaucoma is a condition which results from an increase in the volume of fluid in the eye and usually manifests itself by an increase in intraocular pressure. Thus, measurement of this pressure is done to detect the onset of glaucoma.
Generally there are two types of tonometry: applanation and indentation. Applanation tonometry involves the flattening of a portion of the cornea while indentation tonometry involves indenting the cornea with some shape other than a flat surface. The human eye is made up of tissue and fluid which are mostly water. Because water is substantially incompressible, the pressure required to applanate or indent the cornea will be indicative of the intraocular pressure, assuming a normal ocular rigidity.
Several types of applanation tonometers have been developed and used over the years, including contact and non-contact devices. The contact-type devices generally attempt to measure the pressure within the eye by applying an external force which is then opposed by the pressure of the fluid in the eye. Schemes have been developed for transmitting pressure within the eye to some fluid or gas contained within the tonometer. An example of such a system is the Pneumatonometer in which an equilibrium between gas pressure and intraocular pressure across a thin membrane is sought. Other devices attempt to measure the pressure by allowing deflections in membranes or crystals when the device is pressed against the eye. The Mackay-Marg tonometer and the Computon microtonometer are examples of such devices.
Non-contact tonometry normally uses a stream of air to flatten a portion of the cornea while a light source is reflected off of that flattened portion. The force of the airstream required to flatten the cornea is indicative of the intraocular pressure.
Both contact and non-contact tonometers have many drawbacks. The most obvious drawbacks to the majority of the contact tonometers are the risk of damage to the eye and the reliability of the measurement obtained. The drawbacks to the non-contact tonometer are well known and include cost, the level of skill required for operation and the reliability of the measurement.